1. Field of the Invention
The present invention relates to a tunnel junction sensor in a tunnel junction head, and more particularly, to biasing the orientation of the magnetic moment in the free layer in the tunnel junction head using a current field generated by a tunnel current.
2. Description of the Related Art
A read head employing a read sensor may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive, an air bearing surface (ABS) of the combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode, the resistance of the read sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a current is conducted through the read sensor, resistance changes cause potential changes that are detected and processed as playback signals in processing circuitry.
One type of read sensor is a tunnel junction sensor. The details of tunnel junction have been described in a commonly assigned U.S. Pat. No. 5,650,958 to Gallagher et al., which is incorporated by reference herein. The tunnel junction sensor is a device comprised of two ferromagnetic layers (i.e., the pinned and free layers) separated by a thin barrier layer and is based on the phenomenon of spin-polarized electron tunneling. The typical tunnel junction sensor uses free and pinned layers, such as NiFe or CoFe, with a non-magnetic barrier layer therebetween that is thin enough to permit quantum mechanical sense current tunneling to occur through the barrier layer between the free and pinned layers. The pinned layer has a magnetic orientation pinned by exchange coupling with a pinning layer wherein the pinning layer is made of antiferromagnetic material with magnetic spins oriented in a predetermined direction. The tunneling phenomenon is electron spin dependent, making the magnetic response of the tunnel junction sensor a function of the relative orientations and spin polarization of the conduction electrons between the free and pinned layers. Ideally, the magnetic moment orientation of the pinned layer should be pinned 90.degree. to the magnetic moment orientation of the free layer, with the magnetic moment of the free layer being free to respond to external magnetic fields such as fields from a rotating magnetic disk. In the absence of any external fields acting on the free layer, the magnetic moment of the free layer is parallel to the direction of the pinned layer, due to a ferromagnetic coupling therebetween.
From the above it becomes apparent that what is needed is a way of biasing the magnetic moment of the free layer such that it is normal to the magnetic moment orientation of the pinned layer in a tunnel junction head in the absence of the external field.